Cancer treatment has made great progress in recent years. Many new therapies are becoming available, and many more patients are treated. Cancer is characterized by unrestricted cell growth; many cancer therapies work by inhibiting cell division. Since normal cells do not divide after maturation, inhibition of cell division primarily affects the cancer cells and this has been a focus of drug development. However, other cells are also affected by antitumor drugs to different degrees and cancer therapies are often extremely toxic to patients. There is also great variability in the efficacy of treatments. Some drugs are more effective than others for certain patients, for certain cancers, or at certain stages of treatment. Often, combinations of drugs with varying dosages are necessary for efficacious treatment, requiring considerable experimentation to optimize drugs and doses.
Among the agents affecting the therapeutic benefits of cancer drugs are the multi-drug resistance transporter (MDR), also called p-glycoprotein (PGP) or ABCB 1. It has recently become clear that MDR is one of a family of such transporters. Klein I, Sarkadi B, Varadi A. An inventory of the human ABC proteins. Biochim. Biophys. Acta 1999; 1461:237–62. The expression of these proteins can be highly variable. MDR is not expressed in all cancer cells, and may be present at variable levels. The expression of MDR affects drug efficacy by altering drug accumulation at the tumor. Because of the variable efficacy of antitumor drugs caused by this and other factors, a critical part of therapeutic monitoring involves determining the drug location in the body, its half-life, and the range of mechanisms that limit its effectiveness.
Accumulation of a drug reflects the net balance over time of influx (delivery to the tumor) and efflux (removal from the tumor). Influx and efflux are equally important, but recent research has focused upon a series of transport proteins that function as efflux pumps for taxanes, anthracyclines, and other drugs. There are many reasons why a tumor may not be sensitive to a particular drug. However, the first parameter to evaluate is accumulation of drug by the tumor. If the drug doesn't accumulate in the tumor, there won't be an effect. Adequate accumulation is always necessary for drug activity. Thus, a method to determine accumulation of a drug in the tumor could be the first step in therapeutic decision-making for the drug.
Traditional approaches to the determination of drug uptake and retention (drug accumulation in the tumor) have been invasive and most frequently require obtaining a biopsy from the patient. In addition to the discomfort and risks associated with biopsy procedures, only a small sample of tissue is typically obtained, which may not be representative of the entire region.
Taxanes
One class of drugs which has proved particularly useful in the treatment of cancer, including solid tumors such as breast cancer, has been the taxanes. Taxanes are diterpenoid compounds with a complex taxane ring as the nucleus. The taxane paclitaxel (I) (Taxol®) was initially isolated from yew bark, although the compound may now be prepared synthetically. A modification of the side chains of paclitaxel has yielded another clinically effective compound, docetaxel (II). Other taxanes have also been developed, and are at various stages of preclinical and early clinical testing. These analogs include differences in functional groups attached to the main baccatin nucleus, as well as different side chains attached at the C-13 position.
                (I) Paclitaxel: R1=R2=acetate, R3=Ph        (II) Docetaxel: R1=OH, R2=acetate, R3=OC(CH3)3         
Taxanes, like the vinca alkaloids and colchicine, work by interfering with microtubules, thereby inhibiting mitosis. Taxanes antagonize disassembly of microtubules, by promoting tubulin polymerization, inducing microtubule bundles to form. This leads to arrest of mitosis, and ultimately to cell death. The rate of cell death is proportional to concentration of drug and length of time of administration. Taxanes are highly insoluble and are commonly administered in a solution of surfactants and other vehicles such as ethanol. (Hardman, J. G. and Limbird, L. E., (eds) Goodman and Gilman's The Pharmacological Basis of Therapeutics, Chapter 51, McGraw-Hill, New York, 1996)
The taxane class of anticancer drugs has demonstrated remarkable activity. Docetaxel and paclitaxel, the first two approved drugs in this class, have already altered the standard treatments for breast, lung, and ovarian tumors. Crown J, O'Leary M. The taxanes: an update. Lancet 2000; 355(9210):1176–8. Burris H A 3rd. Docetaxel (Taxotere) in the treatment of cancer. Semin. Oncol. 2000; 27 (2 Suppl 3):1–2. The full scope of antitumor activity for docetaxel and paclitaxel is still under active investigation, with new uses emerging for other tumor types. For example, at present there is considerable interest in the use of docetaxel for treatment of prostate cancer. Oh WK, et al. Docetaxel (Taxotere)-based chemotherapy for hormone-refractory and locally advanced prostate cancer. Semin. Oncol. 1999; 26 (5 Suppl 17):49–54; Petrylak D P. Docetaxel (Taxotere) in hormone-refractory prostate cancer. Semin. Oncol. 2000; 27 (2 Suppl 3):24–9. Other molecules in the taxane class are at much earlier stages of clinical testing.
Anthracyclines
Anthracyclines represent another important class of antitumor drugs. Representative anthracycline drugs include doxorubicin (III) and epirubicin (IV). These anthracyclines are leading agents for the treatment of many tumors, notably breast cancer, lung cancer, and sarcomas. Doroshow J H. Anthracylines and anthracenediones. In: Chabner B A and Longo D L, Cancer Chemotherapy and Biotherapy, 2nd Edition, p. 409, Lippincott-Raven, Philadelphia, 1996. Several mechanisms of action have been proposed, based upon the intercalation of the anthracycline molecules with DNA, and subsequent disruptions of cellular functioning.
                (III) Doxorubicin: A=H; B═OH        (IV) Epirubicin: A=OH; B═H        
Other Antitumor Drugs
Because the discovery of anticancer drugs has been well-funded over the last half-century, drugs from a variety of chemical classes are now in routine clinical use. Mitoxantone (V) is an anthracenedione, a chemical class closesly related to anthracyclines. Mitoxantrone was originally developed in an attempt to replace anthracyclines because the anthracenediones have lower cardiac toxicity. However, the antitumor activity of mitoxantrone was generally disappointing and it has a relatively narrow niche in clinical use. Recently, however, mitoxantrone has been demonstrated to substantially reduce the severe pain associated with metastatic prostate cancer, and has changed the management of this large group of cancer patients. Tannock I F, Osoba D, Stockler M R, Ernst D S, Neville A J, Moore M J, Armitage G R, Wilson J J, Venner P M, Coppin C M, Murphy K C. Chemotherapy with mitoxantrone plus prednisone or prednisone alone for symptomatic hormone-resistant prostate cancer: a Canadian randomized trial with palliative end points. J. Clin. Oncol. 1996; 14(6):1756–64.
                (V) Mitoxantrone        
Camptothecin is a natural product found in the bark and wood of a Chinese tree. Although it was found to be too difficult for clinical use itself, a number of derivatives have demonstrated clinical activity. Takimoto C H, Arbuck S G. The Camptothecins. In: Chabner B A and Longo D L, Cancer Chemotherapy and Biotherapy, 2nd Edition, p. 463, Lippincott-Raven, Philadelphia, 1996. This class of drugs appears to work by inhibiting the action of topoisomerase I, a key enzyme for the integrity of DNA structure. Irinotecan (CPT-11) was initially approved for treatment of colorectal cancer, and topotecan (VI) initially approved for
                (VI) Topotecanovarian cancer. Both of these two drugs and other camptothecin analogs are acquiring new uses as testing continues. Topotecan has shown activity in lung cancer, and is now also approved for treating these patients. In addition, topotecan is particularly important from the perspective of PET imaging because it is direct-acting (irinotecan is a prodrug that must be activated), and also has limited catabolism, so that the parent molecule is the principal circulating species.        
Determination of Sensitivity or Resistance
Although drugs having excellent antitumor activity e.g. taxanes, anthracyclines and others, have been identified, not all tumors respond to a given therapy. Furthermore, all patients are exposed to the risk of severe, life-threatening toxicity with these drugs whether or not their tumors respond. An important tool for individualizing therapy would be a method to rapidly determine whether a specific tumor will be likely to respond to a particular drug without exposing the patient to toxic levels of the drug.
In addition to avoiding needless toxicity, a rapid determination is also important because of the tendency of tumors to become more difficult to treat with time. Thus, if one treatment can be predicted to be unsuitable, alternative treatments can be explored without waiting months to determine that the first treatment did not work.
In some cases, the cause of treatment failure can not be determined. In other cases, a specific biochemical or molecular mechanism can be ascertained. For example, the tumor cells may be intrinsically sensitive to the drug, but inadequate amounts of drug are accumulated in the tumor. There are multiple reasons for drug accumulation failures, but, regardless of the underlying cause, the empirical demonstration that adequate (or inadequate) drug was accumulated has enormous medical value in terms of treatment selection for individual patients. Determining drug accumulation levels in a specific tumor pre-treatment would thus provide a great value in exploring a wide range of treatment options.
In addition to the benefits for individual patients, determination of drug delivery has benefits for the general patient population and for the process of drug development. General patient populations include a mixture of tumors which are chemosensitive and chemoresistant. The demonstration of a substantial effect is made difficult when the responding tumors are diluted in a pool of nonresponding tumors. Thus, any technique which can find tumors likely to respond before treatment has begun will produce an enriched study population, and greatly decrease the numbers of patients required to test the overall potential benefit of the drug.
Position Emission Tomography (PET) imaging or scanning uses positron emitter labeled tracers. Positrons are positively charged electrons which result from the decay of a proton rich and neutron deficient isotope. These emitters are generally short lived. Most positron emitters are produced in medical cyclotrons or accelerators. The half life of 11C is 20 min and of 18F is 110 minutes. PET cameras have a spatial resolution of several millimeters and can be used to image the entire body.
The concept of trying to find radiolabeled probes that would be ideal for the investigation of one or more transport pumps should not be confused with the concept of measuring accumulation of the specific drug to guide therapy. The literature is filled with research that attempts to define drug transport systems. Many groups are attempting to find the “ideal” probe for each transporter. The emerging problem is an explosion in the numbers of transporters that are being discovered. Thus, attempts to guide therapy with a drug based upon ideal probes is fraught with difficulty and confounded by the multiplicity of transport mechanisms, which will vary from tumor-to-tumor.
Clinical attempts to measure drug accumulation with imaging have been reported, including the use of 11C-verapamil, 11C-daunorubicin, or 99mTc-sesta-MIBI. All have been limited because they only target MDR and have additional difficulties.
Verapamil is known to interact with the MDR efflux pump, and Hendrikse et al. have demonstrated that images can be obtained with 11C-verapamil in rats. Hendrikse N H, de Vries E G, Eriks-Fluks L, van der Graaf W T, Hospers G A, Willemsen A T, Vaalburg W, Franssen E J. A new in vivo method to study P-glycoprotein transport in tumors and the blood-brain barrier. Cancer Res. 1999; 59:2411–6. Their work also showed that modulation of MDR in vivo could be demonstrated in rats with 11C-verapamil. Although 11C-verapamil may be an elegant probe for MDR per se, it is neither structurally nor functionally related to the taxanes, anthracyclines, anthracenediones, camptothecin analogs, or any other approved anticancer drugs. It is also important to recognize that, in humans, there is rapid and extensive catabolism of verapamil. Schomerus M, Spiegelhalder B, Stieren B, Eichelbaum M. Physiological disposition of verapamil in man. Cardiovasc. Res. 1976; 10(5):605–12. Verapamil itself constitutes only a small fraction of the circulating radioactivity, so the interpretation of the total radioactivity signal obtained with PET is problematic.
Also, although daunorubicin has been used as a probe in cell culture, where it is a stable molecule, it is not a stable molecule in the body and imaging with this component is not very useful. In humans, daunorubicin is converted rapidly by carbonyl reductase to its alcohol metabolite, daunorubicinol. Thus, the signal observed via external PET imaging of labeled daunorubicin is a mixture of these two chemical entities, which can complicate the interpretation. The plasma ratio of metabolite-to-parent is about 2.5:1 (Galettis P, Boutagy J, M a D D. Daunorubicin pharmacokinetics and the correlation with P-glycoprotein and response in patients with acute leukaemia Br. J. Cancer 1994; 70(2):324–9), so it is possible that the metabolite is the species primarily being imaged. However, it is also possible that tissue uptake is more favorable for the parent, so a different ratio might be found.
99Tc-Sestamibi, which is routinely used for cardiac imaging, has also been explored for tumor imaging. As in the case for verapamil, neither the structural nor functional properties of sestamibi resemble those for anticancer drugs. The clinical imaging results are mixed. This probe seems to have the ability to detect tumors and monitor response in some clinical settings. Mankoff D A, Dunnwald L K, Gralow J R, et al. Monitoring the response of patients with locally advanced breast carcinoma to neoadjuvant chemotherapy using 99mTc-sestamibi scintimammography. Cancer 1999; 85(11):2410–23. It is reported that the efflux rate of 99mTc-sestamibi correlates with antitumor response, at least for one stage of breast cancer. Ciarmiello A, Del Vecchio S, Silvestro P, et al. Tumor clearance of technetium 99m-sestamibi as a predictor of response to neoadjuvant chemotherapy for locally advanced breast cancer. J. Clin. Oncol. 1998; 16:1677–83. However, in lung cancer, no predictive value was found. Sasaki M, Kuabara Y, Yoshida T, et al. Can 99m-Tc-mibi-SPECT predict the treatment response of lung cancer? J. Nucl. Med. 2000; 41:286P.
The invention provides advantages that were not previously available by providing a non-invasive method for determining potential drug efficacy by measuring actual drug accumulation in tumors. This invention allows for the efficient determination of the potentially most efficacious treatment plan for antitumor therapy by allowing for individualized optimization of drugs and dosage.
The invention offers the additional, previously unrealized advantage of developing individualized antitumor therapies specific to a particular patient with a particular type of tumor in a particular stage of development.
The invention also provides a method for determining the effectiveness of particular drugs as treatment for particular cancers over a broad range of the population.